Aui-1814  Datepicker does not interact with tab key.

Hey Jon

Apart from add tab key into hideon, I changed triggerShowEvent from 'click' to 'focus' so that the datepicker can be poped up by focus of tab key.

This could changed the original design, but when I refer to the datepicker in Jquery for example, I believe the trigger for datepicker in it is 'focus',  not 'click'. Please see http://jqueryui.com/datepicker/

One side effect of using 'foucs' is that IE below 9 not support event 'foucs', so the datepicker cannot be poped up anyway in those browsers. So this may effect some customers using IE8 even if they always encounter some problem when using IE8.

In the https://github.com/liferay/alloy-ui/blob/master/src/aui-datepicker/tests/unit/js/tests.js, I don't find where to test our new triggers. I simply run yogi test in folder aui-datepicker and it seems test passes. 

```
john@john-Latitude-E6530:~/alloy-ui-master/src/aui-datepicker$ yogi test
yogi [info]  using yogi@0.1.13 on node@0.10.25
yogi [info]  using module: aui-datepicker
yogi [info]  prepping grover tests
yogi [info]  adding tests route
yogi [info]  launching xdr server on port 5001
yogi [info]  listening on: http://127.0.0.1:5000
Starting Grover on 1 files with PhantomJS@1.9.0
  Running 15 concurrent tests at a time.
✔ [aui-datepicker]: Passed: 3 Failed: 0 Total: 3 (ignored 0) (0.156 seconds)
----------------------------------------------------------------
✔ [Total]: Passed: 3 Failed: 0 Total: 3 (ignored 0) (0.156 seconds)
  [Grover Execution Timer] 0.686 seconds
yogi [info]  grover tests complete
```